AMD's "Reverse Hyper-Threading", "Anti-HT", or whatever you'd like to call it, would make it possible to run one thread on multiple cores. This is basically the opposite of HyperThreading, a technology from Intel that enables an operating system to run two threads simultaneously on one core.
It's not really clear how this technology works, the French hardware site X86-Secret heard about it from an AMD developer:
Conscious that K8 architecture could not compete with the next high-speed flagship of Intel, all its hopes is for the moment based on a new "revolutionary" technology (it is our opinion, not it his) on which AMD works in this moment for after-K8. This technology is in fact a kind of anti-HT: There or HyperThreading sought to emulate two virtual processors with a physical processor, it is a goal for AMD of emulating a single virtual processor with two (or several) physical processors.The AMD K10 architecture will be a totally new design because the company can no longer base itself on the K8 core as it fears this old architecture won't be able to compete with Intel's new Core architecture.
Update: Someone reminded me that Intel is working on a similar technology called Mitosis.
Update 2 (July 10, 2006): It looks like the Reverse Hyperthreading was just a hoax.